Brassica is a genus of plants, taxonomically embedded within the Brassicaceae family. It contains many economic important crops that serve as a source of food, but species are also employed in the production of oil. In general, the Brassica genus includes B. napus, B. nigra, B. rapa, B. juncea and B. oleracea. Brassica napus is very important in the production of vegetable oil that is increasingly applied in the fuel industry. Together with Brassica rapa, Brassica nigra and Brassica juncea, Brassica oleracea is a species that plays a very important role in the production of human food. Over time, cultivars originated within B. oleracea amongst which cabbage, Chinese cabbage, cauliflower, collards, broccoli, kohlrabi and Brussels sprouts can be found.
The wild cabbage has been bred into a broad range of different horticultural cultivars that remained sexually compatible. However, their appearance does not show a very high level of phylogenetic similarity. The wide range in differences of morphological characteristics within Brassica oleracea has long been of interest and form the foundation of a cultivars' uniqueness. These include an enlarged inflorescence (cauliflower, broccoli); an enlarged stem (kohlrabi); an enlarged apical bud (cabbage); enlarged lateral buds (Brussels' sprouts).
Breeding of cruciferous vegetables like Brassica oleracea varieties aims at the production of commercial varieties optimally adapted to local growing conditions which allows the grower to maximize the productivity of high quality plants. Many characteristics need to be taken into account during selection which relate to both input as well as output traits. One of the most important input traits in this respect relates to disease resistance, in particular to resistance towards micro-organisms.
Cabbage plants are affected by a wide range of pests and diseases. These threats are therefore high priority objects for cabbage breeders, in order to obtain crops that are vigorous and highly resistant. In absence of resistances in these crops, growers necessarily have to apply agronomic strategies like crop rotation and intercropping in order to reduce pest damage in areas where cruciferous vegetables are grown.
Clubroot is a common, probably the most damaging disease found among cabbages, radishes, turnips and other plants belonging to the Crucifereae family. It is caused by the eukaryotic unicellular organism Plasmodiophora brassicae Wor. The incidence of clubroot in North West Europe, Japan, North America and Australia is estimated to affect around 10% of grown B. oleracea crops. The disseminated spores remain infectious for 15 years. Through addition of calcium and other means to prevent a drop op pH of the soil, the disease pressure may be reduced. Chemical treatment is considered to be ineffective due to environmental regulations or these chemicals are too expensive.
The life cycle of the parasite consists of two main phases. The first phase takes place in the vicinity of root hairs. After germination of the resting spore, the primary zoospore becomes attached to a root hair. By turgor pressure from the spore cyst, a small, wall-less amoeba is injected in the cytoplasm of the host and a primary, multinucleate plasmodium is formed. Through mechanisms of phagocytosis, plasmodia migrate deeper into the root tissue and reach the root cortex. In this second phase, the multinuclear secondary plasmodium will develop and through cell divisions and hypertrophy, gall formation is triggered. Finally, host cells will contain haploid, resisting spores which can be disseminated.
The clubs formed on the roots inhibit the transport of nutrients and water, leading to inhibition of plant growth and an increase of the susceptibility to wilting. Infection by Plasmodiophora brassicae occurs worldwide and it has an extensive host range. Cultural practices and/or chemical treatments have been unsuccessful in protecting crops or have proven to be too expensive. Therefore, the development of resistant cultivars is now considered the most economical and efficient method for the control of clubroot.
However, multiple isolates of Plasmodiophora brassicae exist. This makes breeding of different cultivars of different Brassica oleracea cultivars complex, as different cultivars are infected by different isolates of P. brassicae. Also, field populations of P. brassicae exhibit clear differences in pathogenicity. These populations can be classified with respect to differential pathogenicity. The differential series as classified by Williams (1966) and the European Clubroot Differential set (ECD) by Buczacki are commonly used.
Several genetic sources of resistance have already been described in literature. These relate to monogenic as well as polygenic and recessive as well as dominant types. In general, the monogenic dominant resistances are identified in B. rapa and B. napus. In B. oleracea, in general only polygenic recessive sources have been found. These resistances did not show to be sufficiently resistant to clubroot races and secondly, it has been shown very difficult to transfer the resistance between commercial B. oleracea lines. The transfer of a resistance from B. rapa to B. oleracea has also been shown to be difficult, as the sexual compatibility species of these Brassica species is low.
Resistances to Plasmodiophora brassicae have been identified. However, several arguments are relevant when considering the present invention in the light of the prior art. Firstly, it is important that resistance against Plasmodiophora brassicae is as broad as possible. A broader resistance will provide longer protection, especially in view of the virulent and persistent character of the pathogen causing clubbed roots.
Secondly, the genetic background of the resistance is of important relevance in two ways. On the one hand, it is an advantage when the genetic basis is limited to a small number of genes, preferably one gene. This enables easy transfer of the genetic determinant(s) conferring the resistance to other Brassica plants. On the other hand, with regard to complex genetic relations found within the Brassica genus, it is also an advantage if the genetic determinant(s) conferring the resistance is derived from the same Brassica species, e.g. Brassica oleracea. 
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.